Photovoltaic solar power generation systems are made up of photovoltaic “cells”. Photovoltaic cells are semiconductor devices that convert light directly into energy. When light shines on a PV cell, a voltage develops across the cell, and when connected to a load, a current flows through the cell. The voltage and current vary with several factors, including the physical size of the cell, the amount of light shining on the cell, the temperature of the cell, and external factors.
PV cells exhibit voltage and current characteristics according to their I-V curve, an example of which is shown in FIG. 3. When they are not connected to a load, the voltage across their terminals is their open circuit voltage, Voc; when their terminals are connected together to form a short circuit, they generate their short circuit current, Isc. In both cases, since power is given by voltage multiplied by current, they generate no power. They generate their maximum power when operating at their maximum power point (MPP).
Because power is maximized when cells operate at their MPP, circuits exist that perform Maximum Power Point Tracking (MPPT). These circuits adjust the voltage or current at which cells operate, measure their output power, and seek those voltage and current values at which power output is maximized. Many MPPT algorithms exist.
Cells can be connected in series to increase output voltage, and in parallel to increase current. To generate substantial power at convenient voltages, PV power generation systems are often comprised of strings of series-connected cells, connected in parallel. We refer to these parallel-connected strings herein as “arrays”.
As a convenience, strings of cells are generally packaged in “modules”, made up of one or more series-connected cells, mounted in a frame of aluminum or other material, with a protective covering of glass or other transparent material. Therefore photovoltaic “arrays” often consist of one or more series-connected modules connected in parallel.
An “inverter” is a device that converts DC power to AC power for attachment to the electricity grid. Typically, one inverter is connected to an array of many parallel-connected strings of modules. Most inverters contain MPPT circuitry; max power point tracking of the array is generally done by the inverter.
Since cells generate their maximum power at their maximum power point, it follows that arrays generate their maximum power when all cells comprising the array operate at their maximum power point.
Many conditions exist in today's PV solar power generating systems that cause power to be lost due to cells not operating at their maximum power point. Common conditions include shading, soiling, corrosion and aging. Lacking is an effective, inexpensive means to enable arrays to produce as much power as possible by ensuring that as many cells as possible operate as close as possible to their maximum power point.
We discuss one common cause of power loss here: partial string impairment.
When a fraction of the cells in a series string is impaired, the impaired cells do not generate as much current as the other, normally operating cells. Because they are in series, however, they must accommodate the full current. One way they can do that is to allow a negative voltage to develop. When this happens, the impaired cells are actually dissipating energy, in the form of heat, rather than generating it. This heat dissipation can cause cell damage. To avoid this damage, most modern modules are equipped with bypass diodes. Bypass diodes are diodes connected in parallel with the module's cells, in the direction to allow current to flow from what is normally the negative terminal to the positive. This is illustrated for a 3-module string, in FIG. 4. The effect of the bypass diode is to allow whatever current is being generated by the unimpaired cells to flow around the impaired cells, while limiting the negative voltage drop across the impaired cells to the very small value of the diode's forward bias.
The net effect, then, is that the current through the string is that generated by the unimpaired cells, and the voltage across the string is the sum of the unimpaired cells minus the small bypass diode drop.
In this situation, the performance of the array is affected in two ways, the first of which is simply that the impaired cells no longer generate their maximum power.
The performance of the array is also reduced because other cells in the same string as the impaired cells are forced to operate away from their maximum power point. Strings in an array all operate at the same voltage because they are connected in parallel to one another. Therefore cells connected in series with the impaired cells are forced to operate at a greater than optimal voltage because the overall voltage of their string is the same as that of the other strings.
One approach to solve this problem is to equip each module in a string with a boost circuit that boosts the module voltage as needed. This approach is expensive because a separate boost circuit is needed for each module.
Other solutions have equipped entire strings with a single boost circuit designed to boost the voltage output produced by its string to match that of a particular other “primary” string. These solutions are complex and inflexible, requiring coordination among strings.
A related problem in solar module management exists when single strings of modules are connected to a DC-to-AC inverter. A conventional inverter may include a separate maximum power point tracking circuit for each of several isolated strings connected to the inverter. However this architecture is expensive and inflexible because tracking circuits are provided when the final number of strings connected to the inverter is unknown.
Furthermore, it might be desirable for an array to throttle its power production. Consider a large array supplying power to a small electrical grid. Tremendous strain is placed on the grid when the power supplied to it varies widely and rapidly. If the array happens to be located in an area where clouds come and go frequently and rapidly, the grid can experience such strain multiple times a day. What is needed is a means by which the degree and rate at which power delivery is changed, can be controlled.